Easily machinable, non-magnetic, manganese steel



United States Patent 3,010,823 EASILY MACHINABLE, NON-MAGNETIC,MANGANESE STEEL Howard S. Avery and Henry J. Chapin, Mahwah, N.J.,assignors to American Brake Shoe Company, New York, N.Y., a corporationof Delaware No Drawing. Filed Aug. 7, 1959, Ser. No. 832,163

4 Claims. (Cl. 75123) The properties of standard non-magnetic austeniticmanganese steel with manganese of 11 to 14% and containing 1.0 to 1.4%carbon, are well known. Manganese at these levels contributes anessential austenitic stabilizing effect, and virtually unequalledtoughness and work-hardening characteristics can be achieved by anaustenitizing heat treatment at 1800-1900 F. followed by a Water quench.In industrial applications where impact resistance and resistance towear are required manganese steelis virtually unequalled.

However, manganese steel is virtually unmachinable, and surfacefinishing to a limited extent is restricted to grinding and slow cuttingspeeds with expensive tools and heavy equipment where possible at all.

Austenitic stainless steels are also strong and tough and can be madenon-magnetic with suitable composition control. The so-called 19Cr-9Ni(wrought 304 and cast OFS stainless grades) is usually the mosteconomical for non-magnetic conditions, but the cost of the requiredalloying material is considerably greater than for austenitic manganesesteel, and additionally the alloying materials are considered strategic.While the 19Cr-9Ni stainless steel grades are not particularly easy tomachine, they can he lathe turned, drilled, tapped and threaded, muchmore easily than the standard grade of manganese steel, and for thisreason, in spite of cost and strategic-alloying conditions, have usuallybeen selected in preference to manganese steel for ship-boardinstallations where nonmagnetic properties are essential in castingsthat need to have at least elementary and feasible machining operationsperformed thereon.

There is, therefore, an unfulfilled need for an easily machinablenon-magnetic alloy of low cost and nonstrategic alloy content, and whichcan be strong and tough, and the object of the present invention is toenable this to be achieved in an austenitic manganese steel of novelcomposition.

A fully austenitic 13% manganese steel is virtually nonmagnetic with apermeability at H=24 of about 1.03 or less, but on the other handmanganese steel work hardens so easily that even the best cutting toolswith ordinary equipment ordinarily dulls very rapidly when machining itsattempted. Manganese steel is, therefore, generally typed asnnmachinable; however, it can be cut to a limited extent withcontinually resharpened cemented carbide and so-called cobalt high-speedtools, but even in this instance heavy, rigid equipment continually ingood condition is required coupled with a surface speed not exceeding30-40 surface feet per minute (s.f.m.). It can be ground, and this isthe standard surface finish treatment used as a substitute for what ma-'chining operations are equivalent in result thereto. Drilling andtapping large diameter holes is possible with special equipment, butsuch machining operations are practically impossible for small holessuch as one-quarter inch in standard manganese steel; and While smalldiameter holes are sometimes diflicult to drill with a Wrought 304 gradestainless this can be done as a practical matter.

It has been found in accordance with the present invention thatmanganese steel can be made commercially machinable by conjointly usingexceptionally high manganese contents of the order of 20%, loweringcarbon appreciably below the standard minimum of 1.0%, and addingbismuth in small amounts. Even more superior results are obtained by thefurther addition of sulfur. Advantageously, the alloy is heat treatedand quenched in the usual fashion to assure an austenitic structure ofnon-magnetic character where this is important as in shipboardapplications, and this will of course provide the usual exceptionallevels of toughness and work hardening. Furnace gases, however, mayinduce a thin magnetic skin. In situations requiring a high degree ofnonmagnetic character, this skin should and can be easily ground ofi orotherwise removed. However, a further unexpected result under thepresent invention is that non-magnetic character can be achieved in theas-cast alloy as will be shown.

In applications where exceptional levels of toughness are not essential,the usual heat treatment need not be resorted to since machinability andnon-magnetic character are attained with as-cast alloys under thepresent invention. Moreover, it has been found that alloys of thepresent invention containing bismuthare also nonmagnetic in the as-castcondition after cold working.

Preferably, the manganese content of the present alloy is held betweenabout 19% and 20%, carbon between about 0.4% to 0.6% and recoverablebismuth in the alloy about 0.2% to 0.4%. Sulfur, and nickel plus copperor either one alone, maybe used in small amounts for reasons to beexplained, but these are not essential for the ultimate desiredmachinability. Based on extrapolations of tests and properties, andexperiences, the alloy of the present invention falls within thefollowing scope at least:

EXAMPLE 1 Ingredient: Percent by weight Carbon 0.3-0.8

Manganese 17-22 Bismuth 0.1-1.0 Nickel 0-3 Copper 0-3 Sulfur 0-0.15 IronBalance 7 1 Except for contaminants and impurities and the usualrefining additives such as silicon and deoxidation agents.

are classified as strategic. This type of stainless steel was used asthe basis for comparative data on machinability in the present instance,and as will be set forth below the alloy of the present invention isequal to or superior thereto both in machinability and mechanicalproperties. Briefly, it was found that a manganese steel analyzingessentially about 20% manganese, 0.6% carbon, and 0.3% bismuth can belathe turned at 265 surface feet per minute (s.f.m.), threaded, drilledand tapped. Force requirements and friction in machining are well belowthose of standard Hadfield manganese steel. After toughening by a waterquench from about 1900 F., the alloy has Izod impact resistance of 55foot pounds or above, yield strength from 35,000 to 45,000 p.s.i.,tensile strength above 87,000 p.s.i. and elongation 40% or more. 7

In arriving at the ultimate discovery of machinability and adequatemechanical properties attained by raising manganese, lowering carbon andadding bismuth, numerous heats'were run for testing various additionagents suspected as possibly contributing to the achievement of amachinable manganese steel. These included lead, selenium, zirconium andsilver, but each of these was found objectionable or ineffective instandard manganese steels at about 13% manganese, 1% carbon levels. A

. bismuth addition to a standard manganese steel improved machinabilitybut only at slow speeds of the lathe, and the over-all results for suchbismuth addition, particularly for fast lathe speeds, were notacceptable. By lowering carbon to sub-standard levels and raisingmanganese above standard levels, a bismuth addition resulted inmachinability superior to the stainless grade and friction substantiallylower than the stainless grades. Low friction is' desirable, of course,and in this instance is a measureof the resistance to rubbing betweenthe chip produced from the part by machining and the face of the tool.Friction consumes energy and induces heating and therefore undue wear ofthe tool.

In obtaining the machinability. data, a commercial lathe was equippedwith :a dynamometer to measure the cutting force components. Cuts wereperformed with a commercial sintered carbide cut-ting tool, and a fresh,unused cutting edge insert was used for each test run. All inserts hadthe same cutting edge geometry (60 triangular) and were of coursesupported uniformly in the lathe throughout the tests. Test bars werecast from the various heats, :and to assure that. any decarburizedsurface skins did not enter as an unknown variable, each test bar wasfinished to a one-inch diameter.

In other words, the machinability data were obtained under circumstanceswhere the difference in results would be accounted for only by the alloyvariations.

The character of the chip produced by the cut important since it may beof such nature as to interfere With the cutting area. Such undesirablechips are'represented by. continuous chips. that curve back onthemselves without any definite coil pattern, or' in an open helicalflat-backed coil, and: these chips were. observed with standardmanganese steel (:heat 2072) and the st-ain-- less gradesstudied (heats304 and 261). 'Machinable manganese steel alloys of the presentinvention, however, displayed goodchip character, varying from smalldiscontinuo'us curls to tight short spiral cones which do not interferewith or obscure the cutting area and which are ordinarily encountered inconventional easily machinable alloys. 7

.Table I immediately following lists the various heats 4 Table ICOMPARATIVE MACHINABILITY AND FRICTION RATINGS Balance 0, Mn, Si, sub-Heat perperper- Others, percent stan- No. cent cent cent 1 trally STEELTOUGHENED BY WATER QUENCH Machining 0.1 Machining 0.1 out at 39 s.i.rn.cut at 265 s.f.tu. He at Type HF VF F0 HF ,VF F0 304 1 Stainless 2 155260 0. 86 125 210 0. d 220 0. 67 135 280 0. 70 155 295 0. 76 155 370 0.62 85 270 0. 49 145 370 0. 59 275 0. 60 180 395 0. 67 145 295 0. 72 125325 0. 58 145 285 0. 74 125 330 0. 57 130 285 0. 55 150 345 0. 64 130285 0. 67 65 290 0. 37 280 0. 61 310 0. 58 275 0. 67 190 380 0. 73 125280 0. 66 145 350 0. 61 140 285 0. 71 125 275 O. 66 486- 20% Mn+S 95 2500. 57 416- 20% D/IH-l-Bl 42 230 0. 33 40 215 0. 33 003..- 20% MI1+Bi 35225 0. 29 75 225 0. 51 007-. Bi+S+Ni+Gu 30 0.37 35 175 0. 34

, 1 Mill annealed and cold finished. 2 Rough. 3 Welds to cutting tool.

STEEL AS CAST VF: Vertical Machining 0.1 Out at 265 s.f.m.

Heat

N 0. Type Hori- Vertical Friction zontal force, coeffiforce, lbs. clentlbs.

. Table '11 immediately following sets" forth mechanical 'propertiesof'the present alloys together with magnetic permeability data at H .24,and like properties for rat 5 standard manganese steel and the stainlessgrades are included for comparison yield and tensile data are in p.s.i.with yield at 0.2% ofiset.

Table II MACHINABLE MANGANESE STEELS Chemical analysis of heats Heat No0, per- Mn, per- Si, per- Ni, per Cu, per- Bi, per- S. percent cent centcent cent cent cent Properties of machinable manganese steel alloysHeat- Ultimate Elong. Red. Hard- Mag. Izod Heat No treat- Yield tensilein2, area, ness, perm. impact ment percent percent BHN Standard 13%manganese and stainless grades for comparison Heat- Ultimate Elong. Red.Hard- Mag. Izod Heat No. treat- Yield tensile in 2", area, ness. perm.impact ment percent percent BEN Same magnetic permeability after coldworking. NoTn.-AC:As cast. WQ: Standard treatment 1900 F. for 2 hoursfollowed by water quench. MC=M111 certified annealed and cold finished.v

Based on the foregoing, the two alloys ultimately NOTOHED BAR IMPACTRESISTANCE determined as preferred under the present invention are asfollows: Y All 0y type A B Std. EXAMPLE 2 Izod impact-It. lbs 7548 -105Type 0,7 Mn, Si, 1? 5,7 Ni 7 Cu,7 Bi,7'

" 0 MAGNETIC PERMEABILITY n=24 A 0.55 20.0 0.6 0.05 0.02 0.3 Alloy typeA B Std. B 0.40 20.0 0.6 0.05 0.09 2.2 2 0 0.35 55 As-east 1. 003 1. 003l Maidmurn. 'lt ugghfineg bydwater quene 1. 883 1. 882

or ar ene 1. '1. These upon further testing are found to have thefollowing properties, yield being at 0.2% offset:

Table III TENSILE PROPERTIES AT ROOM TEMPERATURE Yield Ult. Elong Red.Hard- Heat treat- Alloy strength, tens. in 2, area, ness, ment, F.

type p.s.i. strength, percent percent BHN hrs. quench S1551i 13% 52, 000120, 000 40 35 200 1,900-2-water.

40, 000 93, 000 .g E 9 DD 50, 000 67, 000 6. 0 6 187 As cast FORCEREQUIREMENTS FOR SINGLE POINT LATHE Type B alloy is slightly moreexpensive thantype A, but the machinability characteristics examinedshow 75 superiority over type A, and it appears that use of the type Balloy will result in longer tool life. Both of the above alloys havetensile properties superior to those of mild steel and are adequatelystrong and tough for most engineering applications, and magneticpermeability is quite low either in the toughened condition (austenitizeplus water quench) or in the as-cast condition with or without Workhardening. Moreover, while the stand-.

ard manganese steel toughening treatment is recommended, strength andductility in light sections as-cast are adequate for many usefulapplications not requiring superior toughness and hence the standardtoughening treatment is not essential.

One-quarter inch diameter holes can be drilled and' tapped in any of thelow-carbon, high-manganese, bismuth-containing alloys of the presentinvention using ordinary high speed steel tools and standard machineshop equipment.

It appears that at least 1.0% bismuth should be added to the melt toachieve the desired recoverable amount of about 0.3% bismuth in theultimate alloyanalysis However, in our alloys the bismuth analyzed inthe effective alloy can be as low as 0.1%, the carbon can be as low as0.3% or as high as 0.8% (without nickel or copper) and the manganeseshould not be less than about 17% in achieving an austenitic manganesesteel which is The type B alloy is the easiest to machine of all thoseinvestigated. a a

The alloy can be made by melting techniques that are standard. Asuitable mixture of iron or steel, and

. ferro-manganese or metallic manganese, is melted down,

a suitable deoxidizer such as ferrosilicon is added (the .source ofsilicon) and finally metallic bismuth or a suitable alloy or compoundthereof is added in the furnace or ladle. 'In otherwords, alloys of thepresent inven- .tion are produced under standard manganese steel melttechniques, and the objects of the present invention are ,achieved .byfundamental alterations in carbon and manganese contents plus thebismuth addition in the furnace or ladle. The alloy is thenpoured intocastings, ingots or whatever is needed for the next stage ofmanufacture.

It will be manifest from the foregoing, and in particular from thenumerous heats investigated,,that a great deal ofextensive research wasinvolved indeveloping the ultimate analyses .that gave the improvedmachinability results, such culminating in the series of heats 416, 003and 007, and these lead to the ultimate development of the type A andtype B alloys set forth above.

It will therefore be evident that while We disclose and claim specificnovel alloys that render manganese steel fully and acceptablycommercially machinable, for the first time to the best of ourknowledge, variations therefrom but within the realm of what Wedesignate abnormally low carbon levels below the 1.0% standard level andabnormally high levels of manganme above the 13% standard level,together with effective amount of bismuth, are of course possible inachieving specific machinability characteristics or a balance betweenmachinability and desirable mechanical properties under and inaccordance with our invention.

We claim:

1. An easily machinableaustenitic manganese steel alloy which issubstantially non-magnetic both as-cast or when cold worked, said alloycontaining carbon in effective amounts between about 0.3 and 0.8percent, manganese about 17 to 22 percent, and bismuth about 0.1 to 0.5percent.

2. An alloy according to claim '1 containing about 4 percent of anaustenitic stabilizing metal selectedfrom the group consisting ofnickeljand copper.

3. An alloy according to claim 1 containing about 0.4 to 0.6 percentcarbon, about 19 to20 percent manganese, sulfur up to about 0.09percent, and characterized by a friction coefficient of not more thanabout 0.48 and an elongation of not less than about 13 percent for theas cast alloy and a friction coeiiicient of not more than about 0.57 andan elongation of not less than about 39 percent when the alloy isheat-treated at about 1900" F. for about two hours followed by a quench.

4. An alloy according to claim 3 and containing about 2 percent each ofnickel and copper.

References Cited in the file of this patent OTHER REFERENCES Riedrich:Stahl and Eisen, vol. 60, No. 37, September 12, 1940, pages 815-818.Published by Verlag Stahleisen rn.b.H., Dusseldorf, Germany.

1. AN EASILY MACHINABLE AUSTENTIC MANGANESE STEEL AL
 1. AN EASILYMACHINABLE AUSTENITIC MANGANESE STEEL ALLOY WHICH IS SUBSTANTIALLYNON-MAGNETIC BOTH AS-CAST OR WHEN COLD WORKED, SAID ALLOY CONTAININGCARBON IN EFFECTIVE AMOUNTS BETWEEN ABOUT 0.3 AND 0.8 PERCENT, MANGANESEABOUT 17 TO 22 PERCENT, AND BISMUTH ABOUT 0.1 TO 0.5 PERCENT.